The present invention relates to a power supply, in particular to a technique for supplying electric power from a host device to a terminal device.
It has been known that there are systems including a host device (hereinafter it is also simply called “host”) and a terminal device(s) in which electric power is supplied from the host device to the terminal device. For example, in USB (Universal Serial Bus) systems, the host device supplies a DC current from a power supply embedded in an interface to a terminal device through an interface cable (USB cable). In this way, the terminal device can operate without using an AC adapter or the like.
The power supply from the host device to the terminal device in the USB system is explained hereinafter with reference to FIG. 10. As shown in FIG. 10, the USB system 10 includes a host device 20, a terminal device 30, and a USB cable 40 connecting the host device 20 with the terminal device 30. The host device 20 includes a power supply unit 22, a host controller 24, and a connector 26, and the terminal device 30 includes a terminal-device controller 34 and a connector 36.
The power supply unit 22 outputs a voltage to be supplied to the terminal device 30. Hereinafter, the voltage output from the power supply unit 22, i.e., the voltage at a point P1 located at the exit of the power supply unit 22 is referred to as “first voltage V1”.
In the USB standards, a power supply line used to supply electric power from a host to a terminal device is called “VBUS”. For the sake of explanation, the part of the VBUS located on the host side and that located on the terminal-device side are called “host-side VBUS” and “terminal-device-side VBUS” respectively. Further, the part of VBUS located inside the cable is called simply “VBUS”.
As shown in the figure, the power supply unit 22 in the host device 20 applies a first voltage V1 to the host-side VBUS 28.
One end of the host-side VBUS 28 is connected to the power supply unit 22 and the other end is connected to the connector 26. A first end of the connector 26 is connected to the terminal-device-side VBUS 28 and a second end of the connector 26 is connected to a VBUS 42 included in the USB cable 40.
The host controller 24 can control the power supply unit 22 and performs various communications with the terminal-device controller 34 of the terminal device 30 through the USB cable 40. Further, the host controller 24 converts various data to be transmitted to the terminal device 30 into packets and converts packets of various data received from the terminal device 30 into the original date format.
Note that in FIG. 10, only the VBUS 42 of the USB cable 40 is shown and the illustration of other signal lines and the like used for the communication between the host device 20 and the terminal device 30 are omitted.
In the terminal device 30, a first end of the connector 36 is connected to the USB cable 40 and a second end of the connector 36 is connected to the terminal-device-side VBUS 38.
The terminal-device-side VBUS 38 is connected to the connector 36 and thereby supplies electric power into the terminal device 30.
The terminal-device controller 34 performs various controls within the terminal device 30 and performs various communications with the host controller 24 of the host device 20 through the USB cable 40. Further, the host controller 24 converts various data to be transmitted to the host device 20 into packets and converts packets of various data received from the host device 20 into the original date format.
Electric power is supplied from the power supply unit 22 to the terminal device 30 through the host-side VBUS 28, the VBUS 42, and the terminal-device-side VBUS 38.
In the USB standards, a power supply voltage and its permissible deviation as well as the maximum value of a current flowing between the host device 20 and the terminal device 30 are specified. For example, the USB 2.0 standards specify that a voltage at a point immediately in front of the connector 26 (point P3) should be within a range from 4.75 v to 5.25 v. The voltage at the point P3 is hereinafter called “third voltage V3”.
In general, terminal devices are developed so that they operate properly under the condition that the third voltage V3 is within the above-described range, and the sum total of the contact resistance between the connector 26 and the host-side VBUS 28, the resistance of the VBUS 42, the contact resistance between the VBUS 42 and the connector 36, and the contact resistance between the connector 36 and the terminal-device-side VBUS 38 is equal to or smaller than the maximum value that is expected under normal circumstances. Therefore, terminal devices under development are evaluated whether they operate properly under the condition that a voltage that is obtained by measuring a voltage at a predetermined reference point on the terminal-device-side VBUS 38 (point P2 in the figure) is within a range between a value that is obtained by subtracting a margin according to the normally-expected, maximum value of the above-described sum total of the resistances (i.e., resistances from the point P1 to the reference point P2) from the maximum value of the permissible range of the above-described third voltage V3 (4.74 v to 5.25.v) and a value that is obtained by adding a margin according to the normally-expected minimum value of the above-described sum total of the resistances (e.g., 0) to the minimum value of the permissible range of the above-described third voltage V3. The voltage at the point P2 is hereinafter called “second voltage V2”. Further, a range obtained by adding and subtracting a margin to and from the permissible range of the third voltage V3 is called “first reference range” and the permissible range of the third voltage V3 is called “second reference range”.
That is, terminal devices are developed so that they operate properly when the voltage at the reference point P2 on the terminal-device-side VBUS 38 is within the first reference range.